1. Field of Use
The present invention relates to memory systems and more particularly to apparatus and a method for simplifying the construction and expansion of such systems.
2. Prior Art
It is well known to construct memory systems utilizing a number of small memory modules. Normally, a small increment of memory is selected for the module size because it has the advantage of losing less memory space in the event of failure. For further discussion of such systems, reference may be made to U.S. Pat. No. 3,803,560 which is assigned to the same assignee as named herein.
In addition to the above, modular memory systems permit ready expansion of the users memory system capacity or memory space when desired. In order to accommodate such expansion, memory system manufacturers have been required to construct a number of different memory systems having different memory capacities or different size increments of memory. This, in turn, has necessitated the construction of a number of different types of memory units.
One prior art system disclosed in U.S. Pat. No. 4,001,790 assigned to the same assignee as named herein employs an arrangement which can utilize the same memory board construction for any one of a number of memory modules connected to a memory controller.
In the arrangement, one type of memory board (mother board) includes the control logic circuits and another type memory board (daughter board) contains the memory module. It is required that the memory module board be capable of being plugged into any one of a number of different positions.
In this type of arrangement, it is possible to upgrade and expand the memory system utilizing the two types of memory boards mentioned above. Here, the memory increment or memory size corresponds to the capacity of the daughter board.
Due to certain system and/or packaging requirements, it becomes desirable to include both control logic circuits and memory module circuits on a single board. Because of the densities of memory module circuits available, it becomes necessary to provide memory sizes which are less than maximum memory capacity contained on the memory board.
However, each time a purchased memory system constructed with such single board type having less than the maximum memory capacity is desired to be increased or upgraded, the user is required to purchase an additional memory board type which includes the added capacity. Also, for proper maintenance, field personnel are required to have on hand, spares for each different memory board type. To eliminate this, the copending patent application of Chester M. Nibby, Jr. et al titled "Rotating Chip Selection Technique and Apparatus" bearing Ser. No. 921,292 filed on July 3, 1978, discloses a memory board including rotating chip selection circuits which enables construction of a memory subsystem from a single board type. While this arrangement enables the memory subsystem to be upgraded by the addition of the same board type, further expansion beyond the maximum memory capacity of the board still requires utilization of different board types.
Accordingly, it is a primary object of the present invention to provide a technique and apparatus for constructing a low cost modular memory system.
It is a further object of the present invention to provide a memory module low in cost to construct and which can be upgraded beyond its maximum capacity at low cost.